1. Field of the Invention
This invention relates to measurement and inspection systems that use polar coordinate stages to position samples.
2. Description of Related Art
Many measurement and inspection systems mount samples such as semiconductor wafers on X,Y stages. An X,Y stage can move a sample in two independent orthogonal directions X and Y to select an area of the sample for viewing, imaging, or measurement. For example, an X,Y stage can move a wafer to select and position an area of the wafer in the field of view of an imaging system. The travel distances of the X,Y stage in the X and Y directions determine the size of the largest sample that can be inspected from edge to edge, and large samples require large travel distances. Accordingly, inspection systems have become larger to accommodate larger samples, for example, larger diameter semiconductor wafers.
The space required to accommodate the range of motion of an X,Y stage has a width that is equal to or greater than the width of the sample plus the travel distance in the X direction and a length that is equal to or greater than the length of the sample plus the travel distance in the Y direction. FIG. 1 illustrates a system 100 that uses an X,Y stage to position a circular sample 110. System 100 includes an imaging and/or measurement system (not shown) that can be, for example, a video camera, a microscope, an interferometer, a reflectometer, an ellipsometer, an FTIR spectrometer, or any type of spectrophotometer. Such systems typically have a field of view 130 that is much smaller than sample 110. To view the left edge of sample 110, the X,Y stage moves sample 110 to a position 112 where the left edge of sample 110 is in field of view 130. Position 112 is offset to the right from the central position of sample 110 by the radius r of sample 110. A position 116 for viewing the right edge of sample 110 is offset a distance r to the left along the X axis from the central position. Accordingly, the X,Y stage must have a travel distance of 2r along the X axis for edge-to-edge inspection of sample 110. Similarly, the X,Y stage must have a travel distance of 2r along the Y axis between positions 114 and 118, and a minimum area 120 required for an X,Y stage capable of positioning sample 110 for edge-to-edge viewing is about 16*r.sup.2.
Many applications require the sample to be accurately positioned and oriented or at least require accurate information regarding the position and orientation of the sample relative to the X,Y stage. This requirement is common in automated semiconductor manufacturing where the samples are generally round semiconductor wafers. A wafer's position can be accurately determined by rotating the wafer about a rotation axis and monitoring the variation in the perimeter location of the wafer as a function of the rotation. An analysis of the measured perimeter variations can accurately determine the offset from the rotation axis to the center of the wafer. Additionally, the process can identify the orientation of the wafer because most semiconductor wafers have an orientation indicator such as a notch or a flat on its perimeter. An edge detector detects when the flat or notch in the wafer's perimeter rotates past. Examples of such position detector systems, which are often referred to as prealigners, are described in U.S. Pat. No. 4,457,664 of Judell et al., U.S. Pat. No. 5,308,222 of Bacchi et al., U.S. Pat. No. 5,511,934 of Bacchi et al., and U.S. Pat. No. 5,513,948 of Bacchi et al. Prealignment for an X,Y stage requires addition of structure such as a separate prealignment station, from which the wafer is transferred to the X,Y stage after prealignment, or a rotatable sub-stage on the X,Y stage for rotating the wafer.
FIG. 2 illustrates a system 200 using a polar coordinate stage 220 to position sample 110. Polar coordinate stage 220 has a rotatable platform mounted on a linear drive mechanism. The linear drive mechanism moves the platform and a sample along a coordinate axis R, and the platform rotates the sample about the rotation axis of the platform. Polar coordinate stage 220 requires significantly less area when positioning sample 110 for edge-to-edge inspection. In particular, a travel distance r (the radius of the sample) along axis R out to a position 212 is sufficient to center in field of view 130 any radial coordinate .rho. in the range from 0 to r. Rotation of sample 110 then selects an angular coordinate .theta. so that any point on sample 110 can be positioned in field of view 130. Since polar coordinate stage 220 only requires one-dimensional linear motion and half the travel distance of an X,Y stage, the polar coordinate stage takes much less area than an X,Y stage requires. In particular, a polar coordinate stage needs an area of about 6*r.sup.2, which is less than 40% of the area that an X,Y stage requires.
A disadvantage of a polar stage is the portion of sample 100 in field of view 130 generally appears to rotate when the stage rotates sample 100 to move from one inspection location to another. Thus, different areas appear to have different orientations when an operator or machine vision software views the sample through an imaging system. Additionally, the speed of movement generally varies from one location to another for any constant stage rotation speed. In some measurement systems, an operator observes an image of a portion of the sample being measured or inspected and controls movement of the sample to select which areas are measured or inspected. With a polar stage, image rotation and variable image motion can easily confuse or disorient the operator when the operator is continuously viewing or inspecting sample 110 and moving the sample from one position to another. Accordingly, systems and methods are sought that provide the area savings of a polar coordinate stage but avoid the confusion of image rotation and variable speeds of motion.